High-breakdown-voltage semiconductor device

ABSTRACT

A high-breakdown-voltage semiconductor device has a first offset layer and a second offset layer the dosage of which is higher than that of the first offset layer. When the gate is in the ON state, the first offset layer functions as a resurf layer. When the gate is in the OFF state, part of the charge in the first offset layer is neutralized by a drain current flowing through an element having a low ON-resistance, however, the second offset layer functions as a resurf layer. When the drain current is [Acm-1], the amount of charge of electrons is q[C], and the drift speed of carriers is υdrift[cms-1], the dosage n2 of the second offset layer is given by n2&gt;= ID/(qυdrift)[cms-2].

BACKGROUND OF THE INVENTION

The present invention relates to a high-breakdown-voltage semiconductordevice.

In general, a high-breakdown-voltage semiconductor device used in a highvoltage driving circuit or the like and a low-breakdown-voltagesemiconductor device used in a low voltage driving circuit or the likeare formed on the same substrate, constituting a power IC. Such a kindof power IC is known widely and used in various applications. Generally,at the output stage of the power IC, a high-breakdown-voltage MOSFET isused as a high-breakdown-voltage semiconductor device. Thehigh-breakdown-voltage MOSFET requires a low ON-resistance.

FIG. 1 is a cross-sectional view of an element structure of thehigh-breakdown-voltage MOSFET. In the high-breakdown-voltage MOSFET, ap-type body layer 2 is selectively formed on a surface of a p-typesemiconductor substrate 1 having a high resistance. An n-type sourcelayer 3 is selectively formed in a surface of the p-type body layer 2.

An n-type offset layer 4 having a high resistance is formed in thatregion of the surface of the p-type semiconductor substrate 1 whichdiffers from the region of the surface thereof in which the p-type bodylayer 2 is formed. A gate electrode 8 is formed on that region of thep-type body 2 which is located between the n-type source layer 3 and then-type offset layer 4, and that region of the offset layer 4 which isadjacent to the above region of the p-type body 2, with a gateinsulating film 6 and a field oxide film 7 interposed between the gateelectrode 8 and the above regions of the p-type body 2 and offset layer4.

In the high-breakdown-voltage MOSFET, an n-type drain layer 5 is formedin a surface of an offset layer 4, and thus the offset layer 4 serves asa so-called resurf (reduced surface field) layer. The resurf layer cankeep the breakdown voltage of the semiconductor device at a high value,and at the same time, restrict the ON-resistance to a low value. FIG. 2shows drain voltage/drain current characteristic curves in relation togate voltages V_(G) of 0V (OFF state) to 5V with respect to thehigh-breakdown-voltage MOSFET.

As seen from FIG. 2, the high-breakdown-voltage MOSFET having the abovestructure can achieve a high breakdown voltage when the gate voltageV_(G) is low, i.e., it is about 1V or less, and the gate is in the OFFstate. However, it cannot achieve a high breakdown voltage when the gatevoltage V_(G) is more than 1V, and the gate is in the ON state.

To be more specific, in the above high-breakdown-voltage MOSFET,equipotential lines are present at a high density on the drain side inthe surface of the n-type offset layer 4, and an electric field in thatend portion of the n-type drain layer 5 which is opposite to the sourcelayer 3 has a high intensity, due to a drain current flowing through theelement when the gate is in the ON state. In other words, part of thepositive space charge of the n-type offset layer 4 is neutralized by thecharge of electrons moving through the n-type offset layer 4.Consequently, the n-type layer 4 does not act as the resurf layer,lowering the breakdown voltage. This problem becomes more remarkablewhen the gate voltage V_(G) is 3V or more which is ½ or more of therated gate voltage.

In such a manner, the breakdown voltage of the abovehigh-breakdown-voltage MOSFET is low when the gate is in the ON state.Thus, the high-breakdown-voltage MOSFET cannot be used in an analogcircuit in which the drain is directly connected to a power source, andthe gate is biased.

When a drain current I_(D) per 1 cm of a channel width is I_(D), theamount of charge of electrons is q (=1.6×10⁻¹⁹C: coulomb), and the driftspeed of electrons is υ_(drift) (=8×10⁶ cm/s), the negative charge ofthe n-type offset layer 4 is I_(D)/(q·υ_(drift))cm⁻². In addition, thegate width is a length of the gate which is measured in a directionperpendicular to the-cross-section of the element structure of theconventional high-breakdown-voltage MOSFET which is shown in FIG. 1.Hereinafter, it is referred to as a channel width.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide ahigh-breakdown-voltage semiconductor device which can achieve a highbreakdown voltage, both when the gate is in the ON state, and when it isin the OFF state.

The first subject matter of the present invention resides in that anoffset layer is formed to have a two-layer structure, i.e., it isdivided into a first offset layer and a second offset layer the dosageof which is higher than that of the first offset layer, such that thefirst offset layer is closer to the source side than the second offsetlayer, and the second offset layer is closer to the drain side than thefirst offset layer.

The second subject matter of the present invention having the abovestructure resides in that, even if part of the charge of the firstoffset layer extending to the source side is neutralized by a draincurrent which flows through an element having a low resistance when thegate is in the ON state, the charge of the second offset layer closer tothe drain side than the first offset layer is made to remain, and thesecond offset layer is made to serve as a resurf layer. By virtue ofthese features, the entire element achieves a low ON resistance and,simultaneously, a high breakdown voltage, both when it is in the ONstate and when it is in the OFF state.

In order to attain the above object, a high-breakdown-voltagesemiconductor device according to the first aspect of the presentinvention comprises:

a semiconductor substrate;

a body layer of a first conductivity type selectively formed on a regionof a surface of the semiconductor substrate;

a source layer of a second conductivity type selectively formed in asurface of the body layer;

a first offset layer of the second conductivity type selectively formedon a region of the surface of the semiconductor substrate which differsfrom the region of the surface of the semiconductor substrate on whichthe body layer is formed;

a second offset layer of the second conductivity type formed on at leasta surface region of the first offset layer;

a drain layer of the second conductivity type selectively formed in asurface of the second offset layer;

a source electrode formed to contact a surface of the body layer and asurface of the source layer;

a drain electrode formed on a surface of the drain layer;

an insulating film formed on a region of the semiconductor substratewhich is located between the source electrode and the drain electrode;and

a gate electrode formed on at least that region of the body layer whichis located between the source layer and the first offset layer, with theinsulating film interposed between the gate electrode and the an regionof the body layer,

wherein when mobility of carriers in a channel of an element is μ[cm²V⁻¹ s⁻¹], a dielectric constant of the gate insulating film is ε [Fcm⁻¹], a thickness of the gate insulating film is d[cm], a channellength is L[cm], a threshold voltage is V_(T)[V], and a rated gatevoltage is V_(G)[V], a drain current I_(D) per 1 cm of a channel widthis given by:

I _(D)=(μ·ε)·(V _(G)/2−V _(T))/(4Ld)

 and when an amount of charge electrons is q[C], and a drift speed ofcarriers is υ_(drift)[cm s⁻¹], a dosage n₂ of the second offset layer isexpressed by the following formula:

n ₂ ≧I _(D)/(q·υ _(drift))[cm⁻²].

In the above high-breakdown-voltage semiconductor device, the impurityconcentration of the second offset layer is higher than the impurityconcentration of the first offset layer.

A high-breakdown-voltage semiconductor device according to the secondaspect of the present invention comprises:

a semiconductor substrate;

a body layer of a first conductivity type selectively formed on a regionof a surface of the semiconductor substrate;

a source layer of a second conductivity type selectively formed in asurface of the body layer;

a first offset layer of the second conductivity type selectively formedon a region of the surface of the semiconductor substrate which differsfrom the region of the surface of the semiconductor substrate on whichthe body layer is formed;

a second offset layer of the second conductivity type formed on at leasta surface portion of the first offset layer;

a drain layer of the second conductivity type selectively formed in asurface of the second offset layer;

a source electrode formed to contact a surface of the body layer and asurface of the source layer;

a drain electrode formed on a surface of the drain layer;

an insulating film formed on a region of the semiconductor substratewhich is located between the source electrode and the drain electrode;and

a gate electrode formed on a region of the body layer which is locatedbetween the source layer and the first offset layer, with the insulatingfilm interposed between the gate electrode and the region of the bodylayer,

wherein when a dosage of the first offset layer is n₁, and a dosage ofthe second offset layer is n₂, the following relationship is satisfied:2n₁≦n₂≦4n₁.

It is preferable that the above dosage n₁ be 1.5×10¹² cm⁻² or more and4×10¹² cm⁻² or less.

A high-breakdown-voltage semiconductor device according to the thirdaspect comprises:

a semiconductor substrate;

a body layer of a first conductivity type selectively formed on a regionof a surface of the semiconductor substrate;

a source layer of a second conductivity type selectively formed in asurface of the body layer;

a first offset layer of the second conductivity type selectively formedon a region of the surface of the semiconductor substrate which differsfrom the region of the surface of the semiconductor substrate on whichthe body layer is formed;

a second offset layer of the second conductivity type formed on at leasta surface portion of the first offset layer;

a drain layer of the second conductivity type selectively formed in asurface of the second offset layer;

a source electrode formed to contact a surface of the body layer and asurface of the source layer;

a drain electrode formed on a surface of the drain layer;

an insulating film formed on a region of the semiconductor substratewhich is located between the source electrode and the drain electrode;and

a gate electrode formed on a region of the body layer which is locatedbetween the source layer and the first offset layer, with the insulatingfilm interposed between the gate electrode and the region of the bodylayer,

wherein:

the breakdown voltage of the semiconductor device is determined by astate of a depletion layer which is formed in the first offset layerwhen a reverse voltage is applied between the drain layer and the bodylayer, and a voltage of 0V is applied to the gate electrode; and

the breakdown voltage of the semiconductor device is determined by thestate of a depletion layer which is formed in the second offset layerwhen a reverse voltage is applied between the second offset layer andthe body layer, and a gate voltage having the same polarity as a drainvoltage to be applied to the drain electrode is applied to the gateelectrode.

In the high-breakdown-voltage semiconductor device according to thethird aspect, the impurity concentration of the second offset layer ishigher than an impurity concentration of the first offset layer.

According to the first to third aspect, the second offset layer containscarriers at a density corresponding to an impurity concentration of thesecond offset layer, even when a reverse voltage is applied between thesecond offset layer and the body layer, and a voltage of 0V is appliedto the gate electrode.

It is preferable that the semiconductor substrate is of the secondconductivity type.

However, the semiconductor substrate may be of the first conductivitytype, and its impurity concentration may be lower than the impurityconcentration of the first offset layer.

It is preferable that the gate electrode extend over at least a part ofthe surface of the first offset layer, and a thickness of a region ofthe insulating film which is located between the gate electrode and anend portion of the second offset layer be greater than a thickness of aregion of the insulating film which is located between the gateelectrode and an end portion of the first offset layer.

Note that the gate electrode may extend over a part of the surface ofthe second offset layer.

To summarize, in the present invention, when the gate is in the OFFstate, the first offset layer functions as a resurf layer as in aconventional high-breakdown-voltage MOSFET. When the gate is in the ONstate, part of the charge in the first offset layer is neutralized bya-drain current flowing through the element having a low ON-resistance;however, the second offset layer the dosage n₂ of which is higher thanthe dosage n₁ of the first offset layer functions as a resurf layer. Byvirtue of this feature, a low ON-resistance can be achieved and, at thesame time, a high breakdown voltage can be achieved, both when the gateis in the ON state and when it is in the OFF state.

According to the first aspect, the above advantageous effect can beobtained easily and reliably since the formula“n₂≧I_(D)/(q·υ_(drift))[cm⁻²]” is satisfied as mentioned above.According the second aspect, the effect can be obtained more easily andreliably since the formula “2n₁≦n₂≦4n₁” is satisfied.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross-sectional view of an element structure of aconventional high-breakdown-voltage MOSFET.

FIG. 2 is a view showing drain voltage/drain current characteristiccurves of the conventional high-breakdown-voltage MOSFET.

FIG. 3 is a cross-sectional view of an element structure of ahigh-breakdown-voltage MOSFET according to the first embodiment of thepresent invention.

FIG. 4 is a view showing drain voltage/drain current characteristiccurves of the high-breakdown-voltage MOSFET according to the firstembodiment.

FIG. 5 is a view showing that relationship between an elementbreak-down-voltage and a dosage of a second n-type offset layer in thefirst embodiment, which is established when a gate is in the ON state.

FIG. 6 is a cross-sectional view of an element structure of ahigh-breakdown-voltage MOSFET according to a modification of the firstembodiment of the present invention.

FIG. 7 is a cross-sectional view of an element structure of ahigh-breakdown-voltage MOSFET according to the second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention will be explained withreference to the accompanying drawings.

(First Embodiment)

FIG. 3 is a cross-sectional view of an element structure of ahigh-breakdown-voltage MOSFET according to the first embodiment of thepresent invention. In the high-breakdown-voltage MOSFET, a p-type bodylayer 12 is selectively formed on a surface of a p-type semiconductorsubstrate 11 having a high resistance, and an n-type source layer 13having a low resistance is selectively formed on a surface of the p-typebody layer 12.

A first n-type offset layer 14 having a high resistance is formed onthat region of the surface of the p-type semiconductor substrate 11which differs from the region thereof on which the p-type body layer 12is formed. A second n-type offset layer 15 having a lower resistance(higher dosage) than the first n-type offset layer is formed on an innersurface portion of the first n-type offset layer 14.

An n-type drain layer 16 having a lower resistance than the secondn-type offset layer 15 is selectively formed on a surface of the secondoffset layer 15. A gate electrode 19 is formed on that portion of thesurface of the p-type body layer 12 which is located between the n-typesource layer 13 and the first n-type offset layer 14, and on thatsurface portion of the first n-type offset layer 14 which is continuouswith the above portion of the surface of the p-type body layer 12, witha gate insulating film 17 and a field oxide film 18 interposed betweenthe gate electrode and the portion of the surface of the p-type bodylayer 12 and the surface portion of the first n-type offset layer 14.

A source electrode 20 is formed to contact the n-type source layer 13and the p-type body layer 12. A drain electrode 21 is formed on then-type drain layer 16.

The layers and electrodes shown in FIG. 3 are arranged in a stripepattern in a direction perpendicular to the cross section of the elementwhich is shown in FIG. 3. In addition, a plurality of elements eachhaving the stripe-form element structure shown in FIG. 3 are formed inparallel with each other, constituting a high-break-down semiconductordevice.

When the mobility of carriers in a channel of the element is μ[cm²/V·s],the dielectric constant of the gate insulating film 17 is ε [F/cm], thethickness of the gate insulating film 17 is d[cm], the channel length isL[cm], the threshold voltage is V_(T) [V], and the rated gate voltage isV_(G) [V], a drain current I_(D) per 1 cm of a channel width isexpressed by the following equation (1):

I _(D)=(μ·ε)·(V _(G)/2−V _(T))/(4Ld)[A/cm]  (1)

In this case, when the dosage of the second n-type offset layer 15 isn₂, the charge amount of electrons is q[=1.6×10⁻¹⁹C], and the driftspeed of electrons is υ_(drift), [=8×10⁶ cm/s]; the dosage n₂ of thesecond n-type offset layer 15 is set to satisfy the followingrelationship (2) with the charge amount q[C] of electrons and the driftspeed υ_(drift)[cm/s] thereof:

n ₂ ≧ _(D)/(q·υ _(drift))[cm⁻²]  (2)

In the first embodiment, μ=700 [cm²/V·s], ε=3.5×10⁻¹³ [F/cm], d=1.5×10⁻⁶[cm], L=1×10⁻⁴ [cm], V_(T)=1[V], V_(G)=5[V], and the dosage n₂ is 9×10¹²[cm⁻²]. The drift speed of electrons increases as the electric fieldincreases to be saturated to a constant value. The above-mentionedυ_(drift) corresponds to this constant value and the value is 8×10⁶cm/s.

The operation of the high-breakdown-voltage MOSFET having the abovestructure will be explained.

In the high-breakdown-voltage MOSFET, the first n-type offset layer 14function as a resurf layer as in the conventional high-breakdown-voltageMOSFET, and the breakdown voltage is set at a high value as shown inFIG. 4. At this time, the dosage of the second n-type resurf layer 15 isset such that the second n-type resurf layer 15 is not completelydepleted.

On the other hand, when the gate is in the ON state (the gate voltageV_(G)=5V), a high-break-down voltage can be achieved as shown in FIG. 4,although part of the charge of the first n-type offset layer 14 isneutralized by the drain current flowing through the element. This isbecause the second n-type offset layer 15, the dosage of which is higherthan that of the first n-type offset layer 14, serves as a resurf layer.Furthermore, a low ON-resistance can be obtained, in addition to theachievement of a high-breakdown voltage which has no relation to whetherthe gate is in the ON state or in the OFF state.

In such a manner, according to the first embodiment, a low ON-resistancecan be obtained, and simultaneously a high-breakdown-voltage can beachieved when the gate voltage is in the range of 0V to 5V.

FIG. 5 is a view showing that relationship between the dosage n₂ of thesecond n-type offset layer 15 and the breakdown voltage of the element,which is established when the gate is in the ON state. In this case, thedosage n₁ of the first n-type offset layer 14 is 3×10¹² [cm⁻²]. As isseen from FIG. 5, the breakdown voltage greatly increases-when thedosage n₂ falls within 6×10¹² [cm⁻²] to 1.2×10¹³ [cm⁻²]. Therefore, itis preferable that the dosage n₂ satisfy the following relationship:2n₁≦n₂≦4n₁. This is because if n₂ is smaller than 2n₁ (n₂<2n₁), part ofthe charge is neutralized by the drain current. On the other hand, if n₂is great than 4n₁ (4n₁<n₂), the second n-type offset layer 15 is notcompletely depleted, nor does it serve as a resurf layer or function toachieve a high breakdown voltage.

Furthermore, the same result as in FIG. 5 can be obtained, and therelationship 2n₁≦n₂≦4n₁ is satisfied, when the dosage n₁ of the firstn-type offset layer 14 falls within 1.5 to 4×10¹² [cm⁻²]. In this case,n₂ is within 3×10¹² to 1.6×10¹³ [cm⁻²].

As stated above, according to the first embodiment, when the gate is inthe OFF state, the first n-type offset layer 14 functions as a resurflayer, thus achieving a high breakdown voltage, as in the conventionalhigh-breakdown-voltage MOSFET. When the gate is in the ON state, a lowON-resistance can be obtained, and at the same time, a high breakdownvoltage can be achieved regardless of whether the gate is in the ONstate or in the OFF state, although part of the charge in the firstn-type offset layer 14 is neutralized by the drain current I_(D) flowingthrough the element the ON-resistance of which is low. This is becausethe second n-type offset layer 15, the dosage n₂ of which is higher thanthe dosage n₁ of the first n-type offset layer 14, functions as a resurflayer.

The above advantageous effect can be obtained easily and reliably sincethe dosage n₂ is determined to satisfy the following condition (theabove relationship (2)): n₂≧I_(D)/(q·υ_(drift))[cm⁻²]. In this case, itis preferable that the dosage be also optimized by the value of a draincurrent I_(D) to be used.

Furthermore, the above effect can be obtained more easily and reliablyif the dosage n₁ of the first n-type offset layer 14 is set to fallwithin 1.5 to 4×10¹² [cm⁻²], and the dosage n₂ of the second n-typeoffset layer 15 is set to satisfy the relationship 2n₁≦n₂≦4n₁.

In the first embodiment, the second n-type offset layer 15 is formed onthe inner surface of the first n-type offset layer 14. However, as shownin FIG. 6, the thickness of the second n-type offset layer 15 may beincreased such that the lower surface thereof is located deeper than thefirst n-type offset layer 14, if the following condition is satisfied:in the surface of the p-type semiconductor substrate 1, the first n-typeoffset layer 14 is provided adjacent to the p-type body layer 12, andthe second n-type offset layer 15 is located further from the p-typebody layer 12 than that end portion of the first n-type offset layer 14which is adjacent to the p-type body layer 12.

In addition, it is desirable that an insulating film 18 at the positionwhere the end portion of the second n-type offset layer 15 is oppositeto the gate electrode 19 is greater in thickness than the gateinsulating film 17 at the position where the end portion of the firstn-type offset layer 14 is opposite to the gate electrode 19. Further, itis preferable that the end portion of the gate electrode on the drainside extends over the thick insulation film 18. In the above explanationfor the first embodiment, the insulating film 18 is referred to as thefield oxide film 18, which is greater in thickness than the gateinsulating film 17. This structure can relax the electric field strengthbetween the second n-type offset layer 15 and the gate electrode 19,thereby to prevent a leakage current from flowing therebetween.

(Second Embodiment)

FIG. 7 is a cross-sectional view of a high-breakdown-voltagesemiconductor device according to the second embodiment of the presentinvention. With respect to the second embodiment, structural elementsidentical to those in the first embodiment will be denoted by the samereference numerals, and explanation of those structural elements will beomitted.

The second embodiment differs from the first embodiment in the followingpoint: the semiconductor device according to the second embodiment has asemiconductor substrate 11′ which is of an n⁻-type. In this case, thesemiconductor substrate 11′ needs to have a higher resistance than thefirst n-type offset layer 14 to prevent a current from flowing throughthe substrate 11′. As a result, the same effect can be obtained as inthe first embodiment.

Furthermore, with respect to the first and second embodiments, the aboveexplanations are made on the premise that the first conductivity type isa p-type and the second conductivity type is an n-type. However, thefirst and second embodiments are not limited to this. The firstconductivity type and the second conductivity type may be an n-type anda p-type, respectively.

As explained above, according to the present invention, ahigh-breakdown-voltage semiconductor device can be provided which canobtain a low ON-resistance and, simultaneously, achieve a high breakdownvoltage, both when the gate is in the ON state and when it is the OFFstate.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A high-breakdown-voltage semiconductor devicecomprising: a semiconductor substrate; a body layer of a firstconductivity type selectively formed on a region of a surface of thesemiconductor substrate; a source layer of a second conductivity typeselectively formed in a surface of the body layer; a drain layer of thesecond conductivity type selectively formed on the surface of thesemiconductor substrate apart from the body layer; a second conductivitytype layer formed between the body layer and the drain layer, the secondconductivity type layer comprising: a first layer formed on a side ofthe source layer; and a second layer formed on a side of the drain layerand contacting the drain layer; a source electrode formed to contact asurface of the body layer and a surface of the source layer; a drainelectrode formed on a surface of the drain layer; and a gate electrodeformed on at least a region of the body layer which is located betweenthe source layer and the first layer, with an insulating film interposedbetween the gate electrode and the region of the body layer, whereinwhen an amount of charge of electrons is q(C), a drift speed of carriersis υ_(drift) (cm/s), and a drain current per 1 cm of a channel width isI_(D), a dosage n₂ of the second layer is set by the following formula:n ₂ ≧I _(D)/(qυ_(drift))(cm⁻²).
 2. The high-breakdown-voltagesemiconductor device according to claim 1, wherein an impurityconcentration of the second layer is higher than an impurityconcentration of the first layer.
 3. The high-breakdown-voltagesemiconductor device according to claim 1, wherein the second layercontains carriers at a density corresponding to an impurityconcentration of the second layer, even when a reverse voltage isapplied between the drain layer and the body layer, and a voltage undera threshold voltage of the semiconductor device is applied to the gateelectrode.
 4. The high-breakdown-voltage semiconductor device accordingto claim 1, wherein the semiconductor substrate is of the secondconductivity type.
 5. The high-breakdown-voltage semiconductor deviceaccording to claim 1, wherein the semiconductor substrate is of thefirst conductivity type, and an impurity concentration of thesemiconductor substrate is lower than an impurity concentration of thefirst layer.
 6. The high-breakdown-voltage semiconductor deviceaccording to claim 1, wherein the gate electrode extends over at least apart of the surface of the first layer, and a thickness of a region ofthe insulating film which is located between the gate electrode and anend portion of the second layer is greater than a thickness of a regionof the insulating film which is located between the gate electrode andan end portion of the first layer.
 7. A high-breakdown-voltagesemiconductor device comprising: a semiconductor substrate; a body layerof a first conductivity type selectively formed on a region of a surfaceof the semiconductor substrate; a source layer of a second conductivitytype selectively formed in a surface of the body layer; a drain layer ofthe second conductivity type selectively formed on the surface of thesemiconductor substrate apart from the body layer; a second conductivitytype layer formed between the body layer and the drain layer, the secondconductivity type layer comprising: a first layer formed on a side ofthe source layer; and a second layer formed on a side of the drain layerand contacting the drain layer; a source electrode formed to contact asurface of the body layer and a surface of the source layer; a drainelectrode formed on a surface of the drain layer; and a gate electrodeformed on at least a region of the body layer which is located betweenthe source layer and the first layer, with an insulating film interposedbetween the gate electrode and the region of the body layer, whereinwhen a dosage of the first layer is n₁, and a dosage of the second layeris n₂, the following relationship is satisfied: 2n₁≦n₂≦4n₁.
 8. Thehigh-breakdown-voltage semiconductor device according to claim 7,wherein the dosage n₁ of the first layer is in the range of 1.5×10¹²cm⁻² to 4×10¹² cm⁻².
 9. The high-breakdown-voltage semiconductor deviceaccording to claim 7, wherein the second layer contains carriers at adensity corresponding to an impurity concentration of the second layer,even when a voltage is applied between the drain layer and the bodylayer, and a voltage under a threshold voltage of the semiconductordevice is applied to the gate electrode.
 10. The high-breakdown-voltagesemiconductor device according to claim 7, wherein the semiconductorsubstrate is of the second conductivity type.
 11. Thehigh-breakdown-voltage semiconductor device according to claim 7,wherein the semiconductor substrate is of the first conductivity type,and an impurity concentration of the semiconductor substrate is lowerthan an impurity concentration of the first layer.
 12. Thehigh-breakdown-voltage semiconductor device according to claim 7,wherein the gate electrode extends over at least a part of the surfaceof the first layer, and a thickness of a region of the insulating filmwhich is located between the gate electrode and an end portion of thesecond layer is greater than a thickness of a region of the insulatingfilm which is located between the gate electrode and an end portion ofthe first layer.
 13. A high-breakdown-voltage semiconductor devicecomprising: a semiconductor substrate; a body layer of a firstconductivity type selectively formed on a region of a surface of thesemiconductor substrate; a source layer of a second conductivity typeselectively formed in a surface of the body layer; a drain layer of-thesecond conductivity type selectively formed on the surface of thesemiconductor substrate apart from the body layer; a second conductivitytype layer formed between the body layer and the drain layer, the secondconductivity type layer comprising: a first layer formed on a side ofthe source layer; and a second layer formed on a side of the drain layerand contacting the drain layer; a source electrode formed to contact asurface of the body layer and a surface of the source layer; a drainelectrode formed on a surface of the drain layer; and a gate electrodeformed on at least a region of the body layer which is located betweenthe source layer and the first layer, with an insulating film interposedbetween the gate electrode and the region of the body layer, wherein,when a voltage is applied between the drain layer and the body layer,and a voltage under a threshold voltage of the semiconductor device isapplied to the gate electrode, the first layer is completely depleted,and the second offset layer is not depleted or partially depleted. 14.The high-breakdown-voltage semiconductor device according to claim 13,wherein, when a voltage is applied between the drain layer and the bodylayer, and a gate voltage having the same polarity as a drain voltage tobe applied to the drain electrode is applied to the gate electrode,space charges of the second layer remain at least in the second layer,while space charges of the first offset layer are neutralized by a draincurrent.
 15. The high-breakdown-voltage semiconductor device accordingto claim 13, wherein an impurity concentration of the second layer ishigher than an impurity concentration of the first layer.
 16. Thehigh-breakdown-voltage semiconductor device according to claim 13,wherein the second layer contains carriers at a density corresponding toan impurity concentration of the second layer, even when a voltage isapplied between the drain layer and the body layer, and a voltage undera threshold voltage of the semiconductor device is applied to the gateelectrode.
 17. The high-breakdown-voltage semiconductor device accordingto claim 13, wherein the semiconductor substrate is of the secondconductivity type.
 18. The high-breakdown-voltage semiconductor deviceaccording to claim 13, wherein the semiconductor substrate is of thefirst conductivity type, and an impurity concentration of thesemiconductor substrate is lower than an impurity concentration of thefirst layer.
 19. The high-breakdown-voltage semiconductor deviceaccording to claim 13, wherein the gate electrode extends over at leasta part of the surface of the first layer, and a thickness of a region ofthe insulating film which is located between the gate electrode and anend portion of the second layer is greater than a thickness of a regionof the insulating film which is located between the gate electrode andan end portion of the first layer.
 20. A high-breakdown-voltagesemiconductor device comprising: a semiconductor substrate; a body layerof a first conductivity type selectively formed on a region of a surfaceof the semiconductor substrate; a source layer of a second conductivitytype selectively formed in a surface of the body layer; a drain layer ofthe second conductivity type selectively formed on the surface of thesemiconductor substrate apart from the body layer; a second conductivitytype layer formed between the body layer and the drain layer, the secondconductivity type layer comprising: a first layer formed on a side ofthe source layer; and a second layer formed on a side of the drain layerand contacting the drain layer; a source electrode formed to contact asurface of the body layer and a surface of the source layer; a drainelectrode formed on a surface of the drain layer; and a gate electrodeformed on a region of the body layer which is located between the sourcelayer and the first layer, with an insulating film interposed betweenthe gate electrode and the region of the body layer, wherein, when avoltage is applied between the drain layer and the body layer, and agate voltage having the same polarity as a drain voltage to be appliedto the drain electrode is applied to the gate electrode, space chargesof the second layer remain at least in the second layer, while spacecharges of the first layer are neutralized by a drain current.
 21. Ahigh-breakdown-voltage semiconductor device comprising: a semiconductorsubstrate; a body layer of a first conductivity type selectively formedon a region of a surface of the semiconductor substrate; a source layerof a second conductivity type selectively formed in a surface of thebody layer; a drain layer of the second conductivity type selectivelyformed on the surface of the semiconductor substrate apart from the bodylayer; a first layer of the second conductivity type formed on thesurface of the substrate between the body layer and the drain layer, thefirst layer being apart from the body layer; a source electrode formedto contact a surface of the body layer and a surface of the sourcelayer; a drain electrode formed on a surface of the drain layer; and agate electrode formed on at least a region of the body layer locatedbetween the source layer and the drain layer, the gate electrode beingapart form the drain layer and the first layer, with an insulating filminterposed between the gate electrode and the region of the body layer,wherein a resistivity of the first layer of the second conductivity isset to be lower than that of the semiconductor substrate and higher thanthat of the drain layer, and when a voltage is applied between the drainlayer and the body layer, and a gate voltage having the same polarity asa drain voltage to be applied to the drain electrode, within a ratedgate voltage range, the rated gate voltage range being a range ofvoltages applicable to the gate electrode in a normal operation andlarger than a threshold voltage of the semiconductor device, is appliedto the gate electrode, a breakdown voltage between the drain layer andthe body layer is larger than one half of a breakdown voltage betweenthe drain layer and the body layer when the gate voltage is set at 0volts.
 22. The high-breakdown-voltage semiconductor device according toclaim 21, wherein, when the voltage is applied between the drain layerand the body layer, and the gate voltage having the same polarity as thedrain voltage to be applied to the drain electrode, within the ratedgate voltage range and larger than the threshold voltage of thesemiconductor device, is applied to the gate electrode, the breakdownvoltage between the drain layer and the body layer is larger than 80% ofthe breakdown voltage between the drain layer and the body layer whenthe gate voltage is set at 0 volts.
 23. The high-breakdown voltagesemiconductor device according to claim 21, wherein, when the voltage isapplied between the drain layer and the body layer, and the gate voltagehaving the same polarity as the drain voltage to be applied to the drainelectrode, within the rated gate voltage range and over the thresholdvoltage of the semiconductor device, is applied to the gate electrode,the breakdown voltage between the drain layer and the body layer islarger than a rated voltage of the breakdown voltage between the drainlayer and the body layer when the gate voltage is set at 0 volts. 24.The high-breakdown voltage semiconductor device according to claim 21,wherein the semiconductor substrate is of the second conductivity type.25. The high-breakdown-voltage semiconductor device according to claim21, when the semiconductor substrate is of the first conductivity type,further comprising a second layer of the second conductivity type on thesurface of the substrate between the body layer and the second layer,and contacting the first layer, an impurity concentration of the secondlayer being lower than that of the first layer.
 26. Ahigh-breakdown-voltage semiconductor device comprising: a semiconductorsubstrate; a body layer of a first conductivity type selectively formedon a region of a surface of the semiconductor substrate; a source layerof a second conductivity type selectively formed in a surface of thebody layer; a drain layer of the second conductivity type selectivelyformed on the surface of the semiconductor substrate apart from the bodylayer; a first layer of the second conductivity type formed on thesurface of the substrate between the body layer and the drain layer, thefirst layer being apart from the body layer; a source electrode formedto contact a surface of the body layer and a surface of the sourcelayer; a drain electrode formed on a surface of the drain layer, and agate electrode formed on at least a region of the body layer locatedbetween the source layer and the drain layer, the gate electrode beingapart from the drain layer and the first layer, with an insulating filminterposed between the gate electrode and the region of the body layer,wherein, when a voltage is applied between the drain layer and the bodylayer, and a gate voltage having the same polarity as a drain voltage tobe applied to the drain electrode, within a rated gate voltage range,the rated gate voltage range being a range of voltages applicable to thegate electrode in a normal operation and larger than a threshold voltageof the semiconductor device, is applied to the gate electrode, abreakdown voltage between the drain layer and the body layer is largerthan one half of a breakdown voltage between the drain layer and thebody layer when the gate voltage is set at 0 volts.
 27. Thehigh-breakdown-voltage semiconductor device according to claim 26,wherein, when the voltage is applied between the drain layer and thebody layer, and the gate voltage having the same polarity as the drainvoltage to be applied to the drain electrode, within the rated gatevoltage range and larger than the threshold voltage of the semiconductordevice, is applied to the gate electrode, the breakdown voltage betweenthe drain layer and the body layer is larger than 80% of the breakdownvoltage between the drain layer and the body layer when the gate voltageis set at 0 volts.
 28. The high-breakdown-voltage semiconductor deviceaccording to claim 26, wherein, when the voltage is applied between thedrain layer and the body layer, and the gate voltage having the samepolarity as the drain voltage to be applied to the drain electrode,within the rated gate voltage range and larger than the thresholdvoltage of the semiconductor device, is applied to the gate electrode,the breakdown voltage between the drain layer and the body layer islarger than a rated voltage of the breakdown voltage between the drainlayer and the body layer when the gate voltage is set at 0 volts. 29.The high-breakdown-voltage semiconductor device according to claim 26,wherein the semiconductor substrate is of the second conductivity type.30. The high-breakdown-voltage semiconductor device according to claim26, when the semiconductor substrate is of the first conductivity type,further comprising a second layer of the second conductivity type on thesurface of the substrate between the body layer and the first layer, andcontacting the first layer, an impurity concentration of the secondlayer being lower than that of the first layer.